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Effect of ring size on benzimidazole acceptor unit on the properties of donor-acceptor-donor type monomers and their electrochromic polymers

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2015
Cansu Ergün, Emine Gül
In this thesis, a series of novel benzimidazole-type acceptor units were designed and synthesized. The target acceptors were benzimidazoles including cyclohexane, cycloheptane and cyclooctane rings. Each acceptor group was coupled with 3,4-ethylenedioxythiophene (EDOT) and dihexyl substituted 3,4-propylenedioxythiophene (PRODOT(C6)) donor groups separately and six donor-acceptor-donor (DAD) type monomers were obtained. The main aim of this thesis is to comperatively elucidate the electrochemical and optical properties of these six DAD type conjugated monomers and their polymers when the ring size in the selected acceptor unit is systematically changed. In EDOT containing monomers, the monomer including cyclohexane ring (E6E) exhibited the strongest intramolecular charge transfer and the most red shifted absorption band (539 nm). The polymer films of the EDOT containing monomers showed similar optical absorption and the band gap values around 1.18 eV. Switching times of the polymers were quite fast and the fastest one was found to be for the polymer including cyclooctane ring (P(E8E)) (0.5 s). Coloration efficiencies were calculated as quite high for all polymers but the highest for P(E8E) (316 C/cm2). All the polymer films exhibited ambipolar doping (both anodically and cathodically dopable) properties (green in the neutral state, gray in the oxidized state and brick-red in the reduced state). In PRODOT containing monomers, absorption maxima were found to be almost the same; 310 / 520 nm. The strongest intramolecular charge transfer were observed in the monomer including cyclooctane ring (P8P). For the polymer films, the lowest band gap value was obtained for the polymer including cylcohexane ring (P(P6P)) (1.29 eV). Switching times of the polymers were quite fast and the fastest response was found in P(P8P) (0.5 s). The coloration efficiencies of all the polymers were calculated as quite high but the highest for P(P8P) (572 cm2/C at its longer wavelength absorption). All the polymer films exhibited ambipolar doping properties (green in the neutral state, gray in the oxidized state and brick-red in the reduced state). After all these studies were finished, the monomers and polymers have been compared to each other in EDOT group and the PRODOT(C6) group, separately. At the end, one of the polymers from each two groups were selected to construct an electrochromic device with poly(3,4-ethylenedioxythiophene) (PEDOT). The device properties were revealed in terms of its switching ability, and color contrast ratio. Devices in both groups showed similar optical properties but the device constructed with PRODOT containing polymer exhibited higher coloration efficiency (189 C/cm2 at its longer wavelength absorption).